Chronic inhibition of Na=/K+ -ATPase activity in superfused rat kidney segments stimulates the synthesis of new active pump sites, a response which appears to be mediated by chronic changes in intracellular levels. Over the course of this response, intracellular Na+ and Ca2+ levels are elevated acutely, but over incubation intervals of 18 hours, the time required for the generation of new Na+/K+ -ATPase enzyme sites, both [Na+i] and [Ca+i] are restored to levels which are not different from controls. Similarly [K+i] levels, though acutely decreased by ouabain treatment are also restored to control levels over 18 hour incubation intervals. Thus the response constitutes an effective cellular "homeostatic response". A defect in this response occurs in suspensions of tubular segments prepared from the kidneys of Dahl salt-sensitive rats, a genetic model of volume expanded hypertension. No response is detectable. [Na+i] continues to rise over 18 hours of ouabain treatment. On the other hand an effective "homeostatic response" is demonstrable in suspensions of tubular segments prepared from the kidneys of Dahl salt-resistant rats, the genetic counterparts of the salt-sensitive animals. The first goal of this proposal is the localization of the defect exposed, within the cellular mechanism of the response. Rates of synthesis and degradation of Na+/K+ - ATPase will be measured in tubular suspensions prepared from the kidneys of both Dahl salt-sensitive and salt-resistant rats. Any defect exposed in the synthetic Na+/K+ -ATPase response, will then be pursued at both the transcriptional and translational level. The second goal of this proposal is to identify the ion species which contribute to the regulation of the rate of Na-K -ATPase synthesis. Changes in specific mRNA levels will be measured in response to a range of intracellular electrolyte manipulations in tubular suspensions from the kidneys of Sprague Dawley rats. Once the potential mediators of the response have been identified, the role played by their possible abnormal regulation in the "homeostatic response" defect in the kidneys of Dahl salt-sensitive rats, will be investigated. The localization of the defect in the response in Dahl salt-sensitive kidneys could well point to defects causal in volume expanded forms of human hypertension.